mercury 800 kiekhaefer blowing fuse??

hi guys, I have a 1971 mercury 800 that blows the inline fuse as soon as I hook up the battery? sometimes it cranks once before blowing the fuse. Im thinking that its shorting out somewhere. and could an under charged battery contribute to this? any help would be much appreciated.

Re: mercury 800 kiekhaefer blowing fuse??

I don,t think a weak battery will cause a fuse to blow out. It might not get spark due to under current but it should spin or try to without blowing the fuse. I would do a complete inspection of the warness making sure it isn,t rubbing the flywheel or corroded wire are not crossing and the grounds re all good and tight. Something is grounding out causing the fuse to blow. I would not continue to test run the starter until this is fixed as it could start a fire and that would not be too cool.

Re: mercury 800 kiekhaefer blowing fuse??

Forget the battery. Not in the fused circuit....is supplies power to the fuse.

A good place to start is to disconnect the battery + terminal thus removing 12v from the engine.

Follow that red wire into the engine and you will probably wind up at one of the large (high current) terminals on the starting solenoid. Besides having power available to go through the solenoid, when closed to start the engine, this is usually a distribution point for other areas.

Since you said what you said, your ignition key controlled circuits, usually purple wires and not part of it. On my engine the down wind side of the fuse goes 2 places: To the trim relays and out the electrical connector to the control box. On my wiring harness the down wind lead from the fuse is red also, but has a purple stripe. For the record, 12v that is switched by the ignition switch is in purple insulation as I previously mentioned.

With a good fuse, Ohm between the smaller red wire from the junction (that is the input to the fuse I mentioned earlier) to the engine block...get to the metal, not paint. For the fuse to blow immediately, you have a heavy short. 12v on a weak battery, divided by 30 amps....30 amps at least, since it blows immediately, gives you a circuit resistance of 0.4 ohms. Giving 0.4 ohms for meter lead resistance (what mine is when I zero the digital meter), would show up on a digital meter as 0.8 ohms. A normal reading would be something no more than 10 amperes through that fuse which would be 1.2 ohms plus the lead res. for a reading of 1.6 ohms.

Easiest thing to disconnect is the electrical wiring harness to the control box, in the engine for your first disconnect and see what happens effort. Once you find your Ohm reading an acceptable value, trace out the last wire you disconnected. If someone has added after market wiring for some other function then you will have more wires to disconnect than I mentioned.

If the wiring harness connector solves the problem you probably have chafing in the harness to the control, or possibly the control internal parts were rubbing on the insulation and wore through it. Again, since the switch being off didn't matter it has to be between the engine connection and the input to the ign switch..............Butttt don't rule out a bad ignition switch. Since ground is one of the inputs to the switch, if it has an internal malfunction, that malfunction could put your 12v input on the ground connection.

One final thought. On a 1971 engine, you could have damaged insulation allowing a wire to contact ground that otherwise wouldn't. So any cracked insulation or pieces missing demand your attention.

Good luck,
Mark

Re: mercury 800 kiekhaefer blowing fuse??

Where is this fuse located and how does it connect to the motor? 1971 Merc 800-4 didn't have any fuses in the electrical harness for the motor.

http://www.maxrules.com/oldmercs/Wiring/1966ona/31.pdf

Maybe the fuse is serving other equipment that is causing the problem??

HTH........ed

Re: mercury 800 kiekhaefer blowing fuse??

Alimuhassen said:

hi guys, I have a 1971 mercury 800 that blows the inline fuse as soon as I hook up the battery? sometimes it cranks once before blowing the fuse. Im thinking that its shorting out somewhere. and could an under charged battery contribute to this? any help would be much appreciated.

Click to expand...

Yep, Ive got the same motor, and no fuse in the motor harness.

Is this a new situation or a new boat to you? What size fuse? Does the motor have power trim?

Re: mercury 800 kiekhaefer blowing fuse??

Thanks for info guys. And I recently bought this boat and was told the motor was a 1971, and it has no serial # on it. But there' is a fuse on the motor, it sits between the starter and fuel pump. I'll take a pic tomorrow.

Re: mercury 800 kiekhaefer blowing fuse??

"But there' is a fuse on the motor, it sits between the starter and fuel pump." Is it just physically there or wired there?

It doesn't matter how many wires are down wind of the fuse, where it is physically located, or where they are routed (internal to the engine or external to power accessories) one of them is shorted. So do as I said above and you will find your smoking gun!

If you don't have an ohmmeter, disconnect all the wires down wind of the fuse, and with a new fuse installed, reconnect the battery lead and one by one replace the wires. As soon as the bad one touches the fused power source the fuse will blow. Then just trace out that wire.

Mark

Re: mercury 800 kiekhaefer blowing fuse??

Thanks Texasmark I'll do that today and hopefully I can figure it out. The fuse is wired there, as soon as the fuse blows, there's no more power at key switch.

Re: mercury 800 kiekhaefer blowing fuse??



I hope that's clear.

Re: mercury 800 kiekhaefer blowing fuse??

Looks to me like the red wire to the top half of the holder is connected to the distribution terminal I mentioned on the starter solenoid power input post. Seeing the solenoid, and seeing the main power lead going back to the terminal and the smaller red wire from that location says that.

Not seeing any more red wires in the pic, I am going to assume that everything is driven off this 12v supply point and fuse. Therefore having no power at the control box is understandable.

The outlet of the fuse holder, red wire, goes up to something that has a red wire exiting which goes around the front of the engine to somewhere. Is this your PTT solenoid power line.....does your engine even have PTT? If so, then what I see must be a junction where maybe there is a third wire that goes straight down and into the electrical engine connector and through that harness goes out to whereever including to the control box.

Regardless, the the easiest thing would be to remove the external connector and try the fuse. Then if it still blows, put the connector back and follow the red wire going around the front of the engine and disconnect the other end of it and try another fuse.

If removing the connector solved the problem you will just have to follow the harness and follow the red wire(s) to wherever it/they goes/go.

Mark

Re: mercury 800 kiekhaefer blowing fuse??

I hate to mention this but mark didn't, you need to do some major wiring repair or replacement before you go to the lake if you hope to get back.

Re: mercury 800 kiekhaefer blowing fuse??

I made reference to removing the electrical harness at the connection on the rt. side of the engine but it dawned on me that I recall seeing 2 large pins in there (years ago) and when the engine was disconnected from the boat, all that it took was that connection, fuel, and shift throttle. No separate wires from the battery to the engine as you have today. So, those two large pins, or 4-don't remember carry hot and cold battery power.

So, unless you provide power to the input side of the fuse with 2ea. 14 or larger ga. wires from the battery, one power to the solenoid terminal and the other ground (-) to the engine block, when you remove the connector you remove power to the engine and the fuse will never blow.

So, in retrospect, if you don't make jumpers, troubleshooting will be harder without using the ohm-multimeter approach.

Course you might get lucky. If you go after that red wire I mentioned going around the front of the engine and disconnect it first, it may be the smoking gun. Otherwise you could have a short in your engine connector (large one on the rt. side), in the harness, or somewhere down circuit. If that's the case an ohm check would be your best bet.

You can get multimeters cheap from several places. I use the ones from Harbor Freight and their little $4.95 yellow digital unit works just fine. Instructions in the box tell you how to use it.

Good Luck,
Mark

Re: mercury 800 kiekhaefer blowing fuse??

Wow, that's quite a mess you've got there. I would bet that the red wire leading around the front of the engine actually goes to the switchbox. The switchbox should have a constant 12V from the battery and a switched 12V from the key switch. The smaller red wire branching off the large battery cable at the solenoid runs through the fuse and branches off to the rectifier and to the switchbox. This is your battery charging circuit and constant 12V to the switchbox. The grey wire running from the rectifier to the bundled cable is the tach signal wire. The yellow (bundled) wire is probably the choke and the red (bundled) wire is probably the 12V to the ignition key. The purple wire then would be the swtiched 12V from the key. Just a best guess from what I can see in the photos.

Looks like you also have the OLD original rectifier mounted in the lower left corner of the first photo.

Perhaps you could explain where some of those wires go to and come from. Its really hard to tell from the photos. There are wires running everywhere.

The connections to the switchbox are as follows from the top down. Your colors will be different.
GREEN - ignition coil
BROWN or NO CONNECTION - original tach signal (no longer used)
RED - constant 12V from battery
WHITE - switched 12V from ignition switch

Here is a wiring diagram of how your motor should be wired.

http://www.maxrules.com/oldmercs/Wiring/1966ona/30.gif

My "GUESS" is that your rectifier is cooked and causing the short.